In the recent past, substantial research and development resources have been directed toward small scale Liquid Crystal Display (LCD) and light valve technologies. These high information content, miniature LCD assemblies enable enhanced availability of graphics, data and video information for employment in high resolution projection displays, such as a reflective LCD projectors, SXGA formats (1,280.times.1,024 pixel resolution) and even HDTV formats (above 1,000 line resolution), or the like.
Reflective LCD projectors, in particular, are highly desirable since they offer the brightness of traditional three-lamp front-projection systems in combination with the high resolution of an LCD panel. At the heart of these optical engines is reflective liquid crystal on crystalline silicon light valve technology which, when combined with sophisticated optical architecture and the appropriate electronic interface, enables very high resolution, high brightness, large screen displays. In one optical engine form, as shown in FIG. 1, full spectrum visible light from an illumination source 13 is directed through a sophisticated polarized beam splitter or prism 14 which separates the light into red, blue and green beam components. Each beam component is then directed into a corresponding red, blue and green reflective LCD panel assembly 20, 20' and 20", each of which is configured to control the amount of light absorbed and reflected. More specifically, each pixel of the pixel array for each LCD panel regulates the amount of red, green, and blue light, respectively, it reflects back into the prism. Subsequently, the prism 14 reintegrates the shades of corresponding red, blue and green light for each pixel to create a single visible color at one convergent location. Collectively, the reintegrated pixels (i.e., from the red LCD panel 20, the blue LCD panel 20' and the green LCD panel 20") create an image which is projected through a lens 15 and onto a screen 16.
Due in part to the complexity of the optical engine 17 and the precise nature of the alignment between the cooperating light valves (i.e., LCD panel assemblies 20, 20' and 20"), the structural and optical coupling to the prism 14 is critical to performance. Current mounting techniques, thus, require completely independent coupling to the prism with independent interconnections to the electronic interfaces. As schematically illustrated in FIG. 1 and as shown in FIGS. 2 an 3, each LCD panel assembly 20, 20' and 20" includes an independent flex circuit device interconnection 18, 18' and 18", respectively, electrically coupling a corresponding respective LCD panel to the electronic interface 19.
One problem associated with the current design of these optical engines is the substantial fabrication costs involved due to the abundance of relatively costly individual interconnections between the LCD panels and the electronic interface. These additional flex circuit device interconnections 18 also increase the space requirements in order to accommodate the individual interconnections. Moreover, manufacture time is increased as well as requiring additional labor resources to connect, position and place the individual flex circuit device interconnections.
Another problem associated with both transmissive and reflective-type LCD panels assemblies is the bowing or warpage of the individual panels caused by residual stresses acting upon the die during operation. This is particularly noticeable in reflective-type LCD panels which have increased flatness requirements due to the nature of the reflective surface of the die. For example, thermal expansion characteristics, as well as lattice mismatching, can generate significant stresses in the underlying substrate material (the silicon), therein causing significant bowing of the mirrored surface. The bowing, which translates to a non-planarity of the surface, causes both (1) a non-uniform thickness of the liquid crystal layer between the bowed reflective surface and the planar transmissive top layer, and (2) variations in the path length of the reflected light from different parts of the element, and of the array. These effects compromise the electro-optic properties of the elements and/or array.
As mentioned, a primary source of these residual stresses originate from the different materials and composites of the LCD panel having different coefficients of expansion. This is best shown in FIGS. 2 and 3 which illustrate a conventional small scale LCD assembly 20 including a die 21 having a pixel array 22. This pixel array 22 is typically composed of rows and columns of electrically conductive pathways each forming an individual pixel (not shown). Each pixel can be individually changed to an "on" condition by selecting the appropriate row and column of pixel array 22. Positioned around or concentrated on one end of the pixel array are a plurality of die bond pads 23 which are internally connected to the pixel array 22 to enable operational control thereof. Selection of the appropriate pixel is controlled by control circuitry, either included within the die 21 or external to the die 21. In either configuration, external control signals may be used to control the functions of the die 21.
A transparent glass plate 24 is typically placed over the die 21 and the pixel array 22, such that a portion of the glass plate 24 overhangs the die 21. The glass plate 24 is usually affixed to die 21 through an adhesive seal 25 which together cooperate to define a sealed volume encompassing the pixel array 22. This sealed volume is then commonly filled with a solution 26 of Polymer Dispersed Liquid Crystals (PDLC). To facilitate grounding of the glass plate 24, a conductive coating (not shown) may be deposited over the undersurface 28 thereof.
The die 21 is typically rigidly or semi-rigidly mounted to a substrate 27 for mounting support and heat conductive dissipation for the die. A conductive adhesive 29 (FIG. 3), such as a conductive epoxy, is generally applied to the undersurface 28 of the die 21 to affix the die directly to the top surface of the substrate 27. Accordingly, a heat conductive pathway is created directly between the die and the substrate to dissipate heat generated by the die.
The flex circuit 18 includes a plurality of flex circuit bond pads 30 which are typically wire bonded to the die bond pads 23 through bonding wires 31. The distal end of flex circuit 18 is coupled to the top surface of substrate 27. Finally, a glob coating 32 is applied to die 21, substrate 27 and the distal end of flex circuit 18. The glob coating 32 (FIG. 3) further normally encapsulates the bonding wires 31 and the die and flex circuit bonding pads 23 and 30 without obscuring a view of the pixel array 22 through the glass plate 24.
As previously indicated, one important aspect in the proper operation of these small scale LCD or light valve assemblies is the maintenance of proper distance uniformity (preferably about 2-4 .mu.m) between the pixel array and the undersurface 33 of the glass plate. Variances in the separation of the glass plates may often times cause the pixel array to function improperly or cause operational failure.
Conventional rigid display device constructions, for example, often warp during operation since the substrate 27, the glass plate 24 and the silicon die 21 are all composed of materials or composites having different coefficients of expansion. The individual components of the LCD assembly, therefore, often expand at different degrees and rates. Further, depending in part upon the construction processes, such as the adhesive curing techniques, significant residual stresses may be induced upon the cell. Eventually, in severe instances, the glass plate 24 may delaminate from the die 21. At a minimum, these internal stresses cause optical defects such as variations in color uniformity and fringes, and variations in the cell gap thickness may cause optical shadows.
This is especially true since the undersurface 28 of the die 21 is typically rigidly affixed or attached directly to the substrate. Moreover, during low temperature conditioning, the glass plate 24 often fractures due to internal stress induced by the substrate which is then transmitted to the glass through the rigidly mounted die. This is especially problemsome at the regions where the adhesive mounts the die to the substrate, and/or where the glob coating contacts the glass. As shown in FIGS. 2 and 3, the glob coating 32 encapsulating the bonding wires 31 often substantially contacts both the die bond pads as well as portions of the transparent plate 24. While the material composing the glob coating 32, such as ultraviolet curable and thermally curable silicones and epoxies, is relatively flexible, it must be sufficiently rigid to provide a protective barrier for the bonding wires 31. Consequently, the glob coating 32 insulating and protecting the bonding wire 31 induces residual stresses between the die 21 and the transparent plate 24.